Manual pipettes of the type in which a piston is moved in a cylinder first in one direction to create a vacuum that enables a liquid to be aspirated into a reservoir and subsequently in the other direction to discharge the liquid are widely in use. The accuracy of such pipettes depends on the ability to aspirate a precise quantity of liquid and to discharge the liquid so that none remains in the pipette reservoir. Often this is difficult because of the formation of a droplet of liquid on the end of the reservoir when liquid is expelled therefrom. While the volume of the droplet may be small, it is to be noted that pipettes are used in transferring volumes measured in microliters, as small as five microliters, and the volume of a droplet may be an appreciable part of the total volume aspirated into the reservoir. In such case, the accuracy of the pipette is severly impaired if the droplet is not transferred with the balance of the aspirated liquid.
This has been recognized in the art and several means have been provided to deal with the problem of droplets remaining in the pipette reservoir. Attention is particularly directed to those pipettes which may be referred to as overblow pipettes. Such pipettes employ a first piston that controls the quantity of liquid aspirated into the pipette reservoir, and a second larger piston that compresses a relatively large volume of air that is then admitted to the reservoir to blow any remaining droplets out of the reservoir and into the receiving vessel at the completion of the discharge stroke. U.S. Pat. Nos. 3,834,590, 3,933,048, and 3,935,734 disclose pipettes of the type to which reference is made.